Battery monitoring system for an implantable medical device

ABSTRACT

An implantable cardiac device, such as a pacemaker, provided with a carbon monofluoride (CF x ) battery. The CF x  battery enables replacement of the typical voltage tripler with a voltage doubler and eliminates the need for a bulky decoupling capacitor. The device includes a precision A/D and voltage monitor to enable more accurate prediction of impending battery end-of-life. Several methods of accurately determining a pending end-of-life of a battery with a flat voltage output throughout discharge, such as a CF x  battery, are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 09/976,311, filed Oct. 11, 2001, and isincorporated herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of implantable medicaldevices and, in particular, to a cardiac stimulation device employing aCF_(x) battery for improved performance and a system for monitoringcharge level of the battery.

BACKGROUND OF THE INVENTION

Implantable medical devices are typically battery powered devices thatare implanted within the patient's body to have therapy available to thepatient on a continuous basis. Battery failure is a particular problemwith these devices as replacement of batteries often requires invasivesurgical procedures. One particularly common type of implantable medicaldevice is an implantable cardiac stimulation device.

Implantable cardiac stimulation devices, such as pacemakers andintra-cardioverter defibrillators (ICD's), are employed to monitorcardiac activity and to provide therapy for patients with a variety ofheart arrhythmias. Typically, these devices include sensors, that senseheart function and physiological parameters, and waveform generation anddelivery systems, that provide electrical waveforms to the heart tocorrect arrhythmias and to ensure that more proper function of the heartis maintained. As the devices are implanted in a patient, it isdesirable that the devices be as small and lightweight as possible inorder to minimize impact on the patient.

Implantable cardiac stimulation devices are typically provided withbatteries to power the monitoring and therapy delivery circuits. Due tothe size constraints, the batteries used in implantable cardiacstimulation devices must be very small in size and yet able to providepower over a long period of time. Once the device is implanted,replacement of batteries typically involves invasive surgery. Hence,there is a strong desire to have small batteries that can providesignificant power output to power the implantable device for extendedperiods of time. Known pacemaker devices typically use lithium iodine(Lil), commonly referred to as lithium batteries. Lithium batteriesoffer relatively high energy storage density and have known, predictabledischarge characteristics.

While lithium batteries are commonly used for implantable cardiacstimulation devices, these batteries require additional circuitry thatdegrade device performance. Specifically, the performancecharacteristics of these batteries often require that additionalcircuitry be added to the device, thereby resulting in consumption oflimited space in the implantable device and also consumption of limitedpower, or this additional circuitry has performance characteristics thatlimit the useful life of the implantable device.

For example, FIG. 1 illustrates a high-level conventional pacemakercircuit diagram of the prior art. Lithium batteries are typically notcapable of providing pacing pulses at increased energy levels. As isshown in FIG. 1, a typical lithium battery and a decoupling capacitorare often connected in parallel to address this problem. The decouplingcapacitor is used to accumulate electrical charge between pacing pulseevents to enable the pacemaker to periodically deliver a pulse of energygreater than a lithium battery is capable of providing directly. Thedecoupling capacitor is continuously charged by the lithium battery andpartially discharged upon a pacing event.

However, known implementations of the decoupling capacitors of therequisite electrical properties occupy a large fraction of the overallvolume of the pacemaker device. As pacemakers shrink in size due toproduct refinement, the size of the capacitor is becoming anincreasingly larger proportion of the total pacemaker volume and ispresenting a limitation to further reduction in the size and weight ofpacemaker devices.

FIG. 1 illustrates another aspect of known pacemaker designs, inparticular, a voltage tripler that is part of the control circuitry forthe pacemaker. The control circuitry performs the basic timing andmonitoring functions of the device and delivers the pacing pulses to thepatient's heart. The voltage tripler increases the voltage delivered bythe lithium battery in order to provide a sufficient potential foreffectively stimulating the heart. A lithium battery will have an opencircuit voltage of approximately 2.7 VDC in a fully charged conditionand approximately 2 VDC near the end of its life and thus requires avoltage tripler to generate the more than 5 VDC required for aneffective pacing pulse. However, the voltage tripler is a source ofoverall system inefficiency as each voltage multiplication incurs somedegree of loss.

A further drawback to the lithium battery will be apparent consideringthe output voltage characteristics illustrated in FIG. 2, which shows atypical voltage vs. charge delivery graph for typical lithium batteries.Multi-chamber pacing is a feature of many pacemaker systems andcomprises supplying pacing stimuli to two different sites in the heartas opposed to pacing a single site. Typical parameters for asingle-chamber pacing system with a lithium iodide battery atbeginning-of-life would be an open circuit voltage of approximately 2.7V (almost 8.1 V after the voltage tripler) with an internal batteryimpedance or equivalent series resistance (ESR) of 300Ω and a singlelead of 500Ω impedance. The voltage across the lead is regulated to beapproximately 5 V and the pulsed current would be approximately 10 mA.Pulses are approximately 1 ms in duration and are applied every second,thus drawing a time averaged current of approximately 10 μA.

Similar multi-chamber pacing to two sites through two 500Ω leadsconnected in parallel would draw a current pulse of approximately 20 mAand a time average current of 20 μA. However, as a lithium battery isdischarged, the open circuit voltage drops while the ESR increases.After supplying approximately 900 mA-h, a typical lithium battery'soutput voltage decreases to approximately 2.4V and the ESR increases toapproximately 10 kΩ. Under these parameters, delivering to two leadswith 20 μA average current pulls the battery output voltage down to 2.2V (2.4 V−20E-6 A×10E3Ω) and thus approximately 6.6 V after the tripler.These battery conditions give marginal performance even with the voltagetripling.

With further use, i.e., further discharge of the lithium battery, theopen circuit voltage continues to decrease and the ESR continues toincrease to approximately 30 kΩμ at EOL. Thus, while a lithium batteryin this condition still has considerable charge remaining, the internalimpedance and voltage at which the charge is available render a lithiumbattery unsuitable for continued multi-chamber pacing. Because of thisfactor, approximately 30-50% of the total typical lithium iodidebattery's capacity is not usable and is wasted.

Thus, the typical lithium/lithium iodine battery currently in use inmany implantable cardiac stimulation devices generally requiresadditional components to deliver the power needed to provide therapy andalso has a limited life span in some implementations. Limited life span,of course, requires more periodic follow up and also requires morefrequent replacement of the device. As stated above, more frequentreplacement of the device is undesirable as it typically requiresinvasive surgical procedures.

A further difficulty that occurs with lithium/lithium iodine batteriesin implantable medical devices is that the internal configuration of thebattery often limits telemetry transfer rates. The power that thelithium battery provides is generally not sufficient to support datatransfer rates from implantable cardiac stimulation devices that are inexcess of approximately 8 Kbaud of data. Typically, the decouplingcapacitors are limited to only providing sufficient power to source thepacing pulses but do not have the capacity for providing sufficientcharge to maintain voltage during a multi-minute, high speedtransmission. This relatively low rate of data transfer thereforerequires longer download periods to obtain data out of the implantabledevice which can be very inconvenient for the patient and the treatingmedical professional as well as consuming additional limited power fromthe battery.

Yet another limitation of lithium iodide batteries is their inability topower atrial anti-tachycardia pacing with a practical longevity.Representative conditions for this situation would be anti-tachycardiapacing at 180 beats per minute at 5 V into 500Ω leads at 1 ms from alithium battery with 30 kΩ ESR. Operation under these parameters wouldpull the effective battery output voltage down to 1.5 V which wouldinhibit normal operation of the microprocessor critical to properoperation of the implantable device.

Other battery technologies exist that may have application inimplantable medical devices, however, these technologies have notgenerally been used due to implementation problems. One such technologyis lithium-carbon monoflouride (LiCF_(1.1)), typically referred to asCF_(x) batteries. CF_(x) batteries have some desirable characteristicsthat show promise for use in implantable medical devices, such aspacemakers. Generally, CF_(x) batteries have twice the mass energydensity as lithium batteries and can thus provide significantly moreelectrical energy as lithium batteries of similar weight. Moreover, theperformance characteristics are comparable to lithium based batteries.

However, the use of improved battery technologies, such as CF_(x)batteries, in critical applications, such as implantable cardiacstimulation devices, has been limited by an inability to determineapproaching end-of-life of the battery accurately and efficiently. Inapplications such as pacemakers and ICDs, it is imperative that thedevice be replaced prior to the battery failing. Battery failure willresult in the device being unable to provide therapeutic stimulation tothe heart which can further result in catastrophic consequences for thepatient.

CF_(x) batteries provide a very stable voltage output throughout theireffective life. As a consequence, CF_(x) batteries do not exhibit outputcharacteristics that are easily modeled to predict the end-of-life.Basically, these batteries generally maintain a substantiallynon-decreasing output voltage until the end-of-life and then their poweroutput drops off very precipitously. This makes detection of approachingend-of-life of the battery extremely difficult. As a consequence, theapplication of CF_(x) batteries to patient critical applications, suchas pacemakers, ICDs and the like, has been extremely limited.

From the foregoing, it will be appreciated that there is a need forimproved batteries for implantable medical devices such as pacemakersand ICDs. To this end, there is a need for a mechanism for detectingend-of-life of battery technologies, such as CF_(x) batteries that haverelatively non-decreasing output voltages over the length of their lifeto allow the use of these batteries in patient critical applications.

SUMMARY OF THE INVENTION

The aforementioned needs are satisfied by the implantable medical deviceof the present invention which, in one aspect, comprises an implantablemedical device that is adapted to provide therapy to a patient thatincludes a battery that has a relatively non-decreasing output voltageuntil end-of-life and a voltage monitor that is monitoring the outputvoltage of the battery to thereby predict the end-of-life of the batteryto enable the implantable medical device to signal the need ofreplacement of the battery.

In one particular implementation, the implantable medical devicecomprises an implantable cardiac stimulation device that includes amicrocontroller and a battery, such as a CF_(x) battery, that provides anon-decreasing output voltage, e.g., a variation of less than 200millivolts over 1800 Milliampere hours of operation until end-of-life ofthe battery. At end-of-life of the battery, the supply voltage drops offrather precipitously, e.g., from approximately 2700 millivolts to 700millivolts in less than 400 Milliampere hours.

The use of a voltage monitor permits the use of batteries, such asCF_(x) type batteries, that have reduced internal resistance, also knownas equivalent series resistance, thereby reducing the need for extracomponents such as de-coupling capacitors in pacemaker applications.Moreover, the reduced internal resistance also permits higher datatransfer rates in excess of 10 k during telemetry and also reduces theneed for voltage triplers as less energy of the battery is beingabsorbed by the battery itself. The system of this invention is alsoparticularly well suited for atrial anti-tachycardia pacing.

In one particular implementation, the voltage monitor includes a bandgap reference and a precise A/D, e.g., a 12 bit A/D that compares thebattery voltage to the band gap reference voltage. This configuration iscapable of monitoring the output voltage of the battery to within 1.0millivolt resolution. The A/D provides a digital output signal to themicrocontroller and the microcontroller evaluates the digital outputsignal to determine if the battery is approaching end-of-life. In oneaspect, the microcontroller evaluates the signal to first determine apredicted end point of the battery and then evaluates the subsequentsignals to determine if the output of the battery is approaching thepredicted end point.

In one implementation, the microcontroller determines if the battery isapproaching end-of-life by ascertaining a beginning-of-life voltage ofthe battery and then determines that end-of-life is approaching when thebattery output voltage has fallen over time back to thebeginning-of-life voltage. In another implementation, themicrocontroller determines that end-of-life is approaching byperiodically measuring the output voltage and determining when a peakoutput voltage has occurred. When measuring voltage, the voltage mayvary slightly due to the current being drawn from the battery at anygiven time. To address this in one implementation, the battery voltageis periodically measured over a pre-selected period of time, e.g., oncea day for a week, and a rolling average is maintained. The rollingaverage can be calculated daily such that the effect of periodicvariations in the daily measurements are reduced. The end-of-life pointof the battery is then predicted by determining when the output voltageof the battery has decreased a pre-selected quantity from the peakvoltage. In yet another implementation, the end-of-life point of thebattery is approximated by calculating a rate of consumption of thestored battery power and then determining a end-of-life point at therate of consumption.

Each of the above-implementations can be used in conjunction with othersto determine a correlated approaching end-of-life point for the battery.Once an end-of-life point is determined for the implantable medicaldevice, the microcontroller of the implantable medical device can set aregister indicating that the end-of-life has occurred or can otherwiseenable an annunciator to advise the patient that it is time to seekreplacement of the battery.

The use of such a monitoring system enables the use of improvedbatteries, such as CF_(x), batteries in patient critical applications.The use of these types of batteries can result in more efficient powerconsumption, improved data transmission and the like. These and otherobjects and advantages of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention may be morereadily understood by reference to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a high-level block circuit diagram of a typical prior artpacemaker employing a lithium battery;

FIG. 2 is a graph of typical discharge characteristics of lithium,silver vanadium oxide, and CF_(x) batteries;

FIG. 3 is a simplified diagram illustrating an implantable stimulationdevice in electrical communication with at least three leads implantedinto a patient's heart for delivering multi-chamber stimulation andshock therapy;

FIG. 4 is a functional block diagram of a multi-chamber implantablestimulation device illustrating the basic elements of a stimulationdevice which can provide cardioversion, defibrillation and pacingstimulation in four chambers of the heart;

FIG. 5 is a block circuit diagram of a pacemaker provided with a CF_(x)battery; and

FIG. 6 is an exemplary flow chart illustrating the battery monitoringprocess implemented by the device of FIGS. 4 and 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forpracticing the invention. This description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

As shown in FIG. 3, there is a stimulation device 10 in electricalcommunication with a patient's heart 12 by way of three leads, 20, 24and 30, suitable for delivering multi-chamber stimulation and shocktherapy. To sense atrial cardiac signals and to provide right atrialchamber stimulation therapy, the stimulation device 10 is coupled to animplantable right atrial lead 20 having at least an atrial tip electrode22, which typically is implanted in the patient's right atrialappendage.

To sense left atrial and ventricular cardiac signals and to provide leftchamber pacing therapy, the stimulation device 10 is coupled to a“coronary sinus” lead 24 designed for placement in the “coronary sinusregion” via the coronary sinus os for positioning a distal electrodeadjacent to the left ventricle and/or additional electrode(s) adjacentto the left atrium. As used herein, the phrase “coronary sinus region”refers to the vasculature of the left ventricle, including any portionof the coronary sinus, great cardiac vein, left marginal vein, leftposterior ventricular vein, middle cardiac vein, and/or small cardiacvein or any other cardiac vein accessible by the coronary sinus.

Accordingly, an exemplary coronary sinus lead 24 is designed to receiveatrial and ventricular cardiac signals and to deliver left ventricularpacing therapy using at least a left ventricular tip electrode 26, leftatrial pacing therapy using at least a left atrial ring electrode 27,and shocking therapy using at least a left atrial coil electrode 28. Fora complete description of a coronary sinus lead, see U.S. patentapplication Ser. No. 09/457,277, “A Self-Anchoring Coronary Sinus Lead”(Pianca et al.), and U.S. Pat. No. 5,466,254, “Coronary Sinus Lead withAtrial Sensing Capability” (Helland), which patents are herebyincorporated herein by reference.

The stimulation device 10 is also shown in electrical communication withthe patient's heart 12 by way of an implantable right ventricular lead30 having, in this embodiment, a right ventricular tip electrode 32, aright ventricular ring electrode 34, a right ventricular (RV) coilelectrode 36, and a superior vena cava (SVC) coil electrode 38.Typically, the right ventricular lead 30 is transvenously inserted intothe heart 12 so as to place the right ventricular tip electrode 32 inthe right ventricular apex so that the RV coil electrode 36 will bepositioned in the right ventricle and the SVC coil electrode 38 will bepositioned in the superior vena cava. Accordingly, the right ventricularlead 30 is capable of receiving cardiac signals, and deliveringstimulation in the form of pacing and shock therapy to the rightventricle.

As illustrated in FIG. 4, a simplified block diagram is shown of themulti-chamber implantable stimulation device 10, which is capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation. While aparticular multi-chamber device is shown, this is for illustrationpurposes only, and one of skill in the art could readily duplicate,eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation.

The housing 40 for the stimulation device 10, shown schematically inFIG. 4, is often referred to as the “can”, “case” or “case electrode”and may be programmably selected to act as the return electrode for all“unipolar” modes. The housing 40 may further be used as a returnelectrode alone or in combination with one or more of the coilelectrodes, 28, 36 and 38, for shocking purposes. The housing 40 furtherincludes a connector (not shown) having a plurality of terminals, 42,44, 46, 48, 52, 54, 56, and 58 (shown schematically and, forconvenience, the names of the electrodes to which they are connected areshown next to the terminals). As such, to achieve right atrial sensingand pacing, the connector includes at least a right atrial tip terminal(A_(R) TIP) 42 adapted for connection to the atrial tip electrode 22.This is used for both anti-bradycardia and anti-tachycardia pacing, asneeded, in the atria.

To achieve left chamber sensing, pacing and shocking, the connectorincludes at least a left ventricular tip terminal (V_(L) TIP) 44, a leftatrial ring terminal (A_(L) RING) 46, and a left atrial shockingterminal (A_(L) COIL) 48, which are adapted for connection to the leftventricular ring electrode 26, the left atrial tip electrode 27, and theleft atrial coil electrode 28, respectively.

To support right chamber sensing, pacing and shocking, the connectorfurther includes a right ventricular tip terminal (V_(R) TIP) 52, aright ventricular ring terminal (V_(R) RING) 54, a right ventricularshocking terminal (R_(V) COIL) 56, and a superior vena cava (SVC)shocking terminal (SVC COIL) 58, which are adapted for connection to theright ventricular tip electrode 32, right ventricular ring electrode 34,the RV coil electrode 36, and the SVC coil electrode 38, respectively.

At the core of the stimulation device 10 is a programmablemicrocontroller 60 which controls the various modes of stimulationtherapy. As is well known in the art, the microcontroller 60 typicallyincludes a microprocessor, or equivalent control circuitry, designedspecifically for controlling the delivery of stimulation therapy and mayfurther include RAM or ROM memory, logic and timing circuitry, statemachine circuitry, and I/O circuitry. Typically, the microcontroller 60includes the ability to process or monitor input signals (data) ascontrolled by a program code stored in a designated block of memory. Thedetails of the design and operation of the microcontroller 60 are notcritical to the present invention. Rather, any suitable microcontroller60 may be used that carries out the functions described herein. The useof microprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

As shown in FIG. 4, an atrial pulse generator 70 and a ventricular pulsegenerator 72 generate pacing stimulation pulses for delivery by theright atrial lead 20, the right ventricular lead 30, and/or the coronarysinus lead 24 via an electrode configuration switch 74. It is understoodthat in order to provide stimulation therapy in each of the fourchambers of the heart 12, the atrial and ventricular pulse generators,70 and 72, may include dedicated, independent pulse generators,multiplexed pulse generators, or shared pulse generators. The pulsegenerators, 70 and 72, are controlled by the microcontroller 60 viaappropriate control signals, 76 and 78, respectively, to trigger orinhibit the stimulation pulses.

The microcontroller 60 further includes timing control circuitry 79which is used to control the timing of such stimulation pulses (e.g.,pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A)delay, or ventricular interconduction (V-V) delay, etc.) as well as tokeep track of the timing of refractory periods, PVARP intervals, noisedetection windows, evoked response windows, alert intervals, markerchannel timing, etc., which is well known in the art.

The switch 74 includes a plurality of switches for connecting thedesired electrodes to the appropriate I/O circuits, thereby providingcomplete electrode programmability. Accordingly, the switch 74, inresponse to a control signal 80 from the microcontroller 60, determinesthe polarity of the stimulation pulses (e.g., unipolar, bipolar,combipolar, etc.) by selectively closing the appropriate combination ofswitches (not shown) as is known in the art.

Atrial sensing circuits 82 and ventricular sensing circuits 84 may alsobe selectively coupled to the right atrial lead 20, coronary sinus lead24, and the right ventricular lead 30, through the switch 74 fordetecting the presence of cardiac activity in each of the four chambersof the heart 12. Accordingly, the atrial (ATR. SENSE) and ventricular(VTR. SENSE) sensing circuits, 82 and 84, may include dedicated senseamplifiers, multiplexed amplifiers, or shared amplifiers. The switch 74determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity.

Each sensing circuit, 82 and 84, preferably employs one or more lowpower, precision amplifiers with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, as knownin the art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the device 10 to deal effectively withthe difficult problem of sensing the low amplitude signalcharacteristics of atrial or ventricular fibrillation. The outputs ofthe atrial and ventricular sensing circuits, 82 and 84, are connected tothe microcontroller 60 which, in turn, are able to trigger or inhibitthe atrial and ventricular pulse generators, 70 and 72, respectively, ina demand fashion in response to the absence or presence of cardiacactivity in the appropriate chambers of the heart 12.

For arrhythmia detection, the device 10 utilizes the atrial andventricular sensing circuits, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 60 by comparingthem to a predefined rate zone limit (i.e., bradycardia, normal, lowrate VT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g., bradycardia pacing, anti-tachycardia pacing,cardioversion shocks or defibrillation shocks, collectively referred toas “tiered therapy”).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device102. The data acquisition system 90 is coupled to the right atrial lead20, the coronary sinus lead 24, and the right ventricular lead 30through the switch 74 to sample cardiac signals across any pair ofdesired electrodes.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the stimulation device 10 to suit theneeds of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape and vector of each shocking pulse to bedelivered to the patient's heart 12 within each respective tier oftherapy.

Advantageously, the operating parameters of the implantable device 10may be non-invasively programmed into the memory 94 through a high-speedtelemetry circuit 100 in telemetric communication with the externaldevice 102, such as a programmer, transtelephonic transceiver, or adiagnostic system analyzer. The high-speed telemetry circuit 100 isactivated by the microcontroller by a control signal 106. The high-speedtelemetry circuit 100 advantageously allows intracardiac electrogramsand status information relating to the operation of the device 10 (ascontained in the microcontroller 60 or memory 94) to be sent to theexternal device 102 through an established communication link 104.

For examples of such devices, see U.S. Pat. No. 4,809,697, entitled“Interactive Programming and Diagnostic System for use with ImplantablePacemaker” (Causey, III et al.); U.S. Pat. No. 4,944,299, entitled “HighSpeed Digital Telemetry System for Implantable Device” (Silvian); andU.S. patent application Ser. No. 09/223,422, filed Dec. 30, 1998,entitled “Efficient Generation of Sensing Signals in an ImplantableMedical Device such as a Pacemaker or ICD” (note: this relates totransfer of EGM data) (McClure et al.), which patents are herebyincorporated herein by reference.

In the preferred embodiment, the stimulation device 10 further includesa physiologic sensor 108, commonly referred to as a “rate-responsive”sensor because it is typically used to adjust pacing stimulation rateaccording to the exercise state of the patient. However, thephysiological sensor 108 may further be used to detect changes incardiac output, changes in the physiological condition of the heart 12,or diurnal changes in activity (e.g., detecting sleep and wake states).Accordingly, the microcontroller 60 responds by adjusting the variouspacing parameters (such as rate, AV Delay, V-V Delay, etc.) at which theatrial and ventricular pulse generators, 70 and 72, generate stimulationpulses.

The stimulation device 10 additionally includes a battery 110 which actsas a primary power source to provide operating power to all of thecircuits shown in FIG. 4. For the stimulation device 10, which employsshocking therapy, the battery 110 must be capable of operating at lowcurrent drains for long periods of time (preferably less than 10 μA) andthen be capable of providing higher current pulses (for capacitorcharging) when the patient requires a shock pulse (preferably, in excessof 100 mA, at voltages above 2 V, for periods of 100 seconds or more).The battery 110 must also have a predictable discharge characteristic sothat elective replacement time can be detected. In this embodiment, thebattery 110 is a carbon monofluoride (CF_(x)) battery. An example of asuitable CF_(x) battery 110 is the model 9424 available from WilsonGreatbatch Ltd. of Clarence, N.Y.

The battery 110 preferably has an output voltage 120 that varies as thebattery 110 delivers electrical charge 122 as illustrated in FIG. 2. Thevoltage 120 of the battery 110 has a beginning voltage 124, a peakvoltage 126, and an end-of-life voltage 130. In this embodiment, thebattery 110 has a beginning-of-life voltage that is approximately 2.4volts and the output voltage 120 then rises to a peak voltage ofapproximately 2.72 volts after delivering approximately 600milliampere-hours of charge 122. Subsequently, the output voltagedecreases to a transition point, corresponding, in this implementation,to the end-of-life point 130, where the output voltage corresponds tothe beginning-of-life voltage 124. It should be appreciated that thebeginning voltage 124 and peak voltage 126 of the battery 110 of thisembodiment are of different values and occur at different points ofcharge 122 delivery. Moreover, for any given battery, the magnitude ofthe beginning-of-life voltage, peak voltage, etc. can vary. However, forCF_(x) batteries, the output voltage typically demonstrates the curveshown in FIG. 2.

In the implementation illustrated in FIG. 2, the CF_(x) battery providesa particularly stable normal output voltage over a significant period ofthe battery's life, e.g., a variation of less than 150 millivolts DCover 1,700 milliampere hours. As is generally illustrated, thisparticular battery provides an output voltage that is approximately 2.7volts DC. The end-of-life point 130 is selected for the battery at apoint where the battery 110 is transitioning between normal output andend-of-life where the output voltage begins to drop off significantly.As is illustrated in FIG. 2, the battery drops more than 1,900millivolts in less than 400 milliampere hours. The relatively stableoutput voltage followed by the transition to a relatively suddendecrease in output voltage requires that additional monitoring beemployed to ensure that the end-of-life point is detected.

The battery 110 of this embodiment is generally semicircular andapproximately 45 mm×22 mm×5 mm. The battery 110 of this embodimentweighs approximately 7.6 g which is almost 7 g, or 47%, less than acomparable lithium-iodide battery. The battery 110 of this embodimentalso has a beginning-of-life (BOL) equivalent series resistance (ESR)132 of approximately 10Ω and the battery's 110 ESR 132 increases to onlyapproximately 1 kΩ near the battery's 110 end-of-life (EOL) 130. Thus,the battery 110 of this embodiment offers a substantially lower ESR 132than the BOL ESR of approximately 300Ω and EOL ESR of approximately 30kΩ for a comparable lithium battery.

The lower ESR 132 of the battery 110 of this embodiment throughout itsuseful life provides an extended longevity of the device 10. Inparticular, the lower internal resistance of the battery 110 results insignificantly less of the power of the battery 110 being dissipated asheat energy in the battery 110. Thus more battery power is available tobe supplied to the component devices of the implantable medical device.

Specifically, with a standard lithium iodine battery as discussed above,the equivalent series resistance is typically so high that a de-couplingcapacitor must be continuously charged in order to provide the neededpower during pacing pulses. Since the equivalent series resistance ofthe CF_(x) battery 110 is so much lower than the equivalent seriesresistance of the standard lithium iodine battery, the need for ade-coupling capacitor is reduced which thereby frees up more space forother components and can allow for more compact implantable devices.

Moreover, the voltage and internal resistance characteristics of thebattery 110 of this embodiment enable effective pacing with a voltagedoubler 142 as illustrated in FIG. 5 as opposed to the voltage triplerof prior art pacers. The voltage doubler 142 doubles the voltage ofsignals provided to the voltage doubler 142 and is fabricated in a knownmanner. As discussed above, the lowered internal resistance of thebattery 110 results in significantly less energy being dissipated asheat and thus allows for more current to be supplied to the leads 20,24, 30. The use of a voltage doubler as opposed to a voltage triplerresults in less dissipation of energy thereby prolonging battery life.

As discussed above in connection with FIG. 2, the end-of-lifecharacteristics of the battery 110 are less easily predicted than priorart battery types. Specifically, a lithium iodine battery and an SVOtype battery have a generally gradual decline in the output voltageversus milliampere hours. The gradual decline thus allows for greaterpredictability for when the battery is reaching end-of-lifenecessitating removal and replacement. However, as is illustrated inFIG. 2, the CF_(x) battery 110 provides a substantially non-decreasingoutput voltage of a significant portion of its life and then itcomparatively suddenly experiences a sharp decline in the output voltageand the end-of-life. It is for this reason that CF_(x) batteries havenot been used in many implantable medical devices, the suddenend-of-life of the battery inhibits prior detecting of its imminentapproach. To address this particular problem, the implantable medicaldevice 10 incorporates a precision detector to detect voltages topredict and determine the approaching end-of-life of the battery tothereby allow the use of CF_(x) batteries in more critical applications,such as implantable cardiac stimulation devices.

FIG. 5 is a schematic block diagram which illustrates the basicconfiguration of the battery monitoring circuit 134, the microcontroller60, the telemetry 100 and the various components allowing for deliveryof pacing pulses to the leads. As is indicated in FIG. 5, the CF_(x)battery 110 is used to power a known band gap reference 136. As isunderstood, the typical band gap reference provides a very stablereference output that is both temperature and input voltage independent.Hence, variations in the output voltage developed by the battery 110does not affect the output of the band gap reference 136 provided thatthe battery output is greater than a preselected minimum. In oneimplementation, the band gap reference 136 will provide an outputvoltage of 1.2 volts provided that the input voltage is greater than 1.5volts. As is indicated in FIG. 2, the preferred CF_(x) battery 110 willprovide an output voltage on the order of approximately 2.6 to 2.7 voltsuntil the end-of-life point is reached. As a consequence, the band gapreference 136 is capable of providing the 1.2 volt signal over theentire period of interest of the battery's life.

As is indicated in FIG. 5, the band gap reference 136 provides an outputsignal to a precise A/D converter 140. The precise A/D converter 140 ispreferably a converter that has 12 bits of resolution and is, therefore,able to provide a digital signal to the programmable microcontroller 60at a resolution of 1.0 millivolts or less. As is also indicated in FIG.5, the precise A/D converter 140 also samples the output voltage of thebattery 110 and it compares the output voltage of the battery 110 to thereference voltage provided by the band gap reference 136. Consequently,the precise A/D converter 140 is able to develop a digital word which isindicative of the difference between the output voltage of the battery110 and the reference voltage provided by the band gap reference 136.This output voltage is then provided to the microcontroller 60 such thatthe microcontroller 60 thereby receives a signal indicative of thedifference between the output voltage of the battery 110 and thereference voltage of the band gap reference 136.

As will be described in greater detail below, the microcontroller 60preferably receives this signal on a periodic basis, e.g., daily, suchthat the microcontroller 60 develops a history of the output voltage ofthe battery 110 that is very precise. By evaluation of this history ofthe output voltage, the microcontroller 60 can both predict anend-of-life point and then determine whether the predicted end-of-lifepoint for the battery 110 is approaching.

As will be discussed in greater detail below, the end-of-life point canbe determined using several different techniques without departing fromthe spirit of the present invention. In one particular implementation,the CF_(x) battery 110 exhibits the output voltage characteristic asillustrated in FIG. 2. At initiation, or beginning-of-life, the battery110 exhibits an initial beginning-of-life output voltage. The battery110 then, over time, provides a somewhat increased output voltage untilsuch time that the battery 110 is beginning to approach end-of-life.Consequently, in this implementation, the microcontroller 60 determinesa predicted end-of-life as being the time period at which the voltagemeasured by the monitoring circuit 134 is equal to the originallymeasured beginning-of-life voltage.

In another implementation, the microcontroller 60 periodically receivesthe signal from the precise A/D converter 140 indicative of the battery110 voltage and it then determines when a peak voltage has occurred. Themicrocontroller 60 then determines that the predicted end-of-life periodoccurs when the signal from the monitoring circuit 134 indicates thatthe output voltage has decreased a preselected amount from the peakvoltage.

In yet another implementation, the microcontroller 60 includes asoftware routine that implements a fuel gauge. The output power providedby the battery 110 is either measured or modeled and, after apredetermined amount of power has been consumed, the microcontroller 60then determines end-of-life has occurred. Naturally, any of the threepreceding implementations can be correlated and used together todetermine the end-of-life of the battery 110 without departing from thespirit of the invention.

In implementations where the voltage is measured, the measurementcircuit 134 periodically provides a signal to the microcontroller 60,e.g., on a daily basis. The microcontroller 60 preferably processes thesignal so as to determine a value indicative of the output voltage forthe battery 110. The actual battery voltage measured on any given daymay vary slightly due to events occurring at about the time themeasurement is taken. For example, successive rapid discharges of thebattery 110 immediately prior to taking the measurement may result in atemporarily lower reading.

The microcontroller 60 is preferably configured to take each of theindividual measurements provided by the measurement circuit 134 andnormalize it with respect to previous and/or subsequently obtainedmeasurements. In one implementation, the microcontroller 60 uses arolling average algorithm where each measurement is averaged togetherwith a pre-selected number of previous and/or subsequent measurements todefine a measurement value for a particular time period. Any number ofdifferent normalization techniques can be used to accommodate temporaryvariations in the measured output voltage of the battery 110 withoutdeparting from the spirit of the invention.

FIG. 6 is a flow chart which illustrates one possible operation of themicrocontroller 60 as it implements a first algorithm for determiningthe end-of-life point of the battery 110. This particular algorithm issimply exemplary and it can use either the beginning-of-life voltage orthe peak voltage to determine a predictive end-of-life point of thebattery 110. In particular, from a start state 200, the microcontroller60 records the beginning-of-life voltage 202 upon power up of theimplantable medical device 10.

Subsequently, the microcontroller 60 continues to periodically sample,in state 204, the output voltage provided by the A/D converter 240. Themicrocontroller 60 is preferably programmed so as to sample a signalindicative of the measured voltage at a periodic rate, e.g., every 12hours or every 24 hours. The microcontroller 60 then preferablynormalizes the sampled measurement in state 205. As discussed above,normalizing the sampled measurement reduces the impact of temporaryvariations in the battery 110 in the determination of end-of-life of thebattery 110 and any of a number of normalization techniques can be usedto obtain a normalized value including using the rolling averagingtechnique described above.

As is indicated in FIG. 6, the microcontroller 60 will also determine,in decision state 206, whether a peak voltage has been measured in state204. The manner in which the microcontroller 60 determines whether apeak voltage has been determined can be done in any of a number of waysincluding evaluating a plurality of previously received normalizedvoltages for a peak followed by a subsequent decline in the normalizedvoltages. In a preferred embodiment, a running daily average of theprevious 30 days is calculated. This daily average is (on a daily basis)compared with all previous 30 day averages that were calculated at least2 weeks previously. The first time that the current 30 day average isfound to have decreased, the highest 30 day average to date is thenlocked in as the peak voltage. In one implementation, the end-of-life ofthe battery 110 is based upon a detected preselected decrease from thepeak normalized voltage.

In another implementation, the time at which the peak normalized voltagehas occurred can also be recorded by the microcontroller 60 such thatthis time component can provide a half-life indication of the remainingbattery life on the assumption that the peak will occur at approximatelyhalf the usage of the battery 110. If the peak normalized voltage hasbeen determined, the microcontroller 60 then records, in state 210, thepeak normalized voltage and a time indication as to when it occurred.The microprocessor 60 then determines the predictive end-of-life point130 of the battery 110. As described above in connection with FIG. 2,the predicted end-of-life point 130 preferably corresponds to thetransition between the relatively non-decreasing normal output voltageof the battery 110 and the region of the battery curve where the battery110 output voltage begins to decline precipitously.

The predicted end-of-life can either correspond to the time period atwhich the normalized sampled output voltage corresponds to thebeginning-of-life voltage measured in state 202 or it can correspond tothe time at which the normalized sampled output voltage has a magnitudethat is a pre-selected amount less than the peak voltage. Hence, in thisimplementation, the microprocessor 60 is determining a voltage valuethat will occur in the future that is indicative of the battery 110approaching an end-of-life condition. The microcontroller 60 continuesto receive samples from the battery monitoring circuit 134 and it thencan compare these values to the voltage value corresponding to thepredicted end-of-life point of the battery 110.

Hence, the microcontroller 60 then determines, in decision state 212,whether the last measured normalized output voltage in state 204 isindicative of the end-of-life of the battery 110 in decision state 212.If it is not, the microcontroller 60 continues to sample the outputvoltage in state 204 as previously described until the end-of-lifevoltage is detected. As discussed above, several differentimplementations can be used to determine end-of-life of the battery 110.

Once the end-of-life point has been determined by the microcontroller 60to have been met, the microcontroller 60 provides an indication of theapproaching end-of-life such that upon subsequent monitoring of theimplantable medical device 10 via the telemetry circuit 100, a treatingmedical professional will be advised by the microcontroller 60 that theend-of-life point of the battery 110 has been detected thereby allowingthe treating medical professional to take appropriate action to replacethe battery 110 or the implantable device 10. The microcontroller 60 canalso be configured to initiate an annunciator to advise the patient thatthe end-of-life of the battery 110 is approaching to thereby induce thepatient to seek assistance from a medical professional. Theseannunciators can either take the form of an audible tone or anelectrical discharge that is sensed by the patient.

It will be further appreciated that, in some circumstance, it may bedesirable to include other end-of-life detecting technologies in orderto ensure that end-of-life of the battery 110 is accurately predictedand detected. For example, the use of a fuel gauge, which eithermeasures or models the amount of power dissipated over time, can also beused by the implantable medical device 10 to determine approachingend-of-life. The fuel gauge routine can be a software implementationused by the microcontroller 60 to model or detect energy output so as toboth provide a prediction as to when the end-of-life transition pointwill occur and whether the energy loss from the battery 110 isindicating that the end-of-life is approaching. This technique can beused in conjunction with the techniques described above in connectionwith FIG. 6 to improve the accuracy of both predicting when end-of-lifewill occur for a particular battery 110 and whether it has occurred.

From the foregoing it will be appreciated that the use of asophisticated battery monitoring technique allows for the use ofimproved batteries such as a CF_(x) battery 110 that has a precipitousend-of-life decline in battery voltage. The use of this type of battery110 reduces the need for a decoupling capacitor in a pacemakerapplication and also allows for higher speed telemetry operation inexcess of 10 k baud as well as anti-tachycardia pacing. Moreover, thesignificantly reduced internal resistance of the battery 110 furtherresults in significantly less energy being consumed internally by thebattery 110 which thereby results in more efficient use of the batterypower and also does not require the use of voltage triplers in pacingapplications.

Although the foregoing description of the preferred embodiment of thepresent invention has shown, described, and pointed out the fundamentalnovel features of the invention, it will be understood that variousomissions, substitutions, and changes in the form of the detail of theapparatus as illustrated, as well as the uses thereof, may be made bythose skilled in the art without departing from the spirit of thepresent invention. Consequently, the scope of the present inventionshould not be limited to the foregoing discussions, but should bedefined by the appended claims.

What is claimed is:
 1. An implantable medical device comprising: a therapy delivery device that is adapted to deliver therapy to an organ of a patient; a controller that controls the delivery of therapy to the organ of the patient; a CF_(x) battery that supplies power to the therapy delivery device wherein the battery has an output characteristic with a substantially non-decreasing output voltage for a first period of time followed by a declining voltage as the battery approaches end-of-life; a battery monitoring circuit, that samples the output voltage of the CF_(x) battery and periodically provides sampled output voltage signals indicative thereof to the controller, wherein the controller determines a predicted end-of-life point of the battery based upon the sampled output voltage signals and wherein the controller monitors the sampled output voltage signals and determines when the sampled voltage signals are indicative of the predictive end-of-life point of the battery.
 2. The device of claim 1, wherein the CF_(x) battery has an output voltage characteristic that has an initial beginning-of-life voltage that increases to a peak voltage and then decreases to the predicted end-of-life point.
 3. The device of claim 2, wherein the controller determines a peak voltage value from the sampled output voltage signals and then determines a predictive end-of-life point of the battery as occurring when the sampled voltage signals have a magnitude that is a pre-selected magnitude less than the magnitude of the peak voltage value.
 4. The device of claim 1, wherein the controller determines a beginning-of-life voltage value based on the sampled voltage output signals and uses the beginning-of-life voltage value to determine the predicted end-of-life point of the battery.
 5. The device of claim 4, wherein the controller determines the predicted end-of-life point as the point at which the sampled output voltage signal has a magnitude corresponding to the magnitude of the beginning-of-life voltage value.
 6. The device of claim 5, wherein the CF_(x) battery has a beginning-of-life voltage of approximately 2.6 volts and a peak voltage of approximately 2.72 volts.
 7. The device of claim 6, wherein the CF_(x) battery provides an output voltage of between approximately 2.6 volts and 2.72 volts for approximately 1700 milliampere hours of operation.
 8. The device of claim 1, wherein the battery monitoring circuit comprises: a band gap reference device coupled to the battery that provides a reference voltage that is substantially temperature independent and substantially voltage independent; and an A/D converter that receives the reference voltage and also receives the output voltage from the battery, wherein the A/D converter sends a digital signal to the controller indicative of the ratio between the output voltage from the battery and the reference voltage.
 9. The device of claim 8, wherein the A/D converter is a 12 bit A/D converter that provides a digital word to the controller that has a resolution of approximately 1 millivolt.
 10. The device of claim 1, wherein the controller periodically receives a plurality of signals from the battery monitoring circuit and wherein the controller normalizes the periodically received values so as to reduce the effect of temporary variations in the output voltage of the battery.
 11. The device of claim 10, wherein the controller averages each received sampled output voltage signal with a pre-selected number of previous sampled output voltage values to obtain a periodic value for evaluation of whether the periodic value is indicative of the predictive end-of-life of the battery.
 12. The device of claim 11, wherein the battery monitoring circuit provides the sampled output voltage signals on a daily basis.
 13. The device of claim 1, wherein the controller sets a flag to indicate that the end-of-life of the battery has been reached such that on subsequent review of the device by a treating medical professional, the treating medical professional is advised of the need to replace the battery in the implantable device.
 14. The device of claim 1, wherein the therapy delivery device comprises at least one lead adapted to be implanted adjacent the heart of the patient so as to provide electrical stimulation to the heart.
 15. The device of claim 1, wherein the controller senses the delivery of therapy by the therapy delivery device and further implements a fuel gauge routine that models battery energy output corresponding to the delivery of therapy and correlates the modeled battery energy output with the sampled voltage signals to determine whether the battery has reached the predicted end-of-life point.
 16. An implantable cardiac stimulation device comprising: at least one lead adapted to be implanted adjacent the heart so as to provide therapeutic stimulation to the heart; a therapeutic stimulation circuit that develops electrical stimulation waveforms to be delivered via the at least one lead to the heart; a controller that controls the delivery of therapeutic electrical stimulation to the heart of the patient; a CF_(x) battery that provides power to the implantable cardiac stimulation device wherein the battery has an output characteristic with a substantially non-decreasing output voltage for a first period of time followed by a declining voltage as the battery approaches end-of-life; a battery monitoring circuit that samples the output voltage of the CF_(x) battery and periodically provides sampled output voltage signals indicative thereof to the controller, wherein the controller determines a beginning-of-life voltage value and a predicted end-of-life point of the CF_(x) battery based at least in part upon the beginning-of-life voltage value and wherein the controller monitors the sampled output voltage signals and determines when the sampled voltage signals are indicative of the predictive end-of-life point of the CF_(x) battery.
 17. The device of claim 16, wherein the implantable cardiac stimulation device comprises a pacemaker.
 18. The device of claim 16, wherein the controller determines a predicted end-of-life point of the CF_(x) battery at a point wherein the output voltage characteristic of the battery is transitioning between a substantially non-decreasing output and a declining voltage.
 19. The device of claim 16, wherein the controller determines the beginning-of-life voltage value based on the sampled voltage output signals and uses the beginning-of-life voltage value to determine the predicted end-of-life point of the CF_(x) battery.
 20. The device of claim 19, wherein the controller determines the predicted end-of-life point as the point at which the sampled output voltage signals have a magnitude corresponding to the magnitude of the beginning-of-life voltage value.
 21. The device of claim 20, wherein the CF_(x) battery has a beginning-of-life voltage of approximately 2.6 volts and a peak voltage of approximately 2.72 volts.
 22. The device of claim 21, wherein the CF_(x) battery provides an output voltage of between approximately 2.6 volts and 2.72 volts for approximately 1700 milliampere hours of operation.
 23. The device of claim 16, wherein the controller determines a peak voltage value from the sampled output voltage signals and then determines a predicted end-of-life point of the CF_(x) battery as occurring when the sampled voltage signals have a magnitude that is a pre-selected magnitude less than the magnitude of the peak voltage value.
 24. The device of claim 16, wherein the battery monitoring circuit comprises: a band gap reference device coupled to the battery that provides a reference voltage that is substantially temperature independent and substantially voltage independent; and an A/D converter that receives the reference voltage and also receives the output voltage from the battery, wherein the A/D converter sends a digital signal to the controller indicative of the difference between the output voltage from the battery and the reference voltage.
 25. The device of claim 24, wherein the A/D converter is a 12 bit A/D converter that provides a digital word to the controller that has a resolution of approximately 1 millivolt.
 26. The device of claim 16, wherein the controller periodically receives a plurality of signals from the battery monitoring circuit and wherein the controller normalizes the periodically received values so as to reduce the effect of temporary variations in the output voltage of the CF_(x) battery.
 27. An implantable cardiac stimulation device comprising: means for delivering therapy to the heart of a patient; means for controlling the delivery of therapy to the heart of the patient; means for supplying power to the implantable cardiac stimulation device wherein the means provides a relatively non-decreasing output voltage over a normal useful life and then transitions into an end-of-life state wherein the output voltage declines from the relatively non-decreasing output voltage; means for monitoring the output characteristics of the battery, wherein the monitoring means periodically samples an output characteristic of the battery and provides an output signal indicative thereof, wherein the means for controlling receives the output signals and uses the output signals to (i) determine a predicted end-of-life point of the means for supplying power based on a beginning-of-life voltage value and (ii) determining whether the output signals are indicative of the means for supplying power having reached the predicted end-of-life point based on the beginning-of-life voltage value.
 28. The device of claim 27, wherein the means for supplying power comprises a CF_(x) battery.
 29. The device of claim 28, wherein the effective series resistance of the battery at beginning-of-life and at the end-of-life is an order of magnitude less than the effective series resistance of an equivalent Li battery used in implantable cardiac stimulation devices.
 30. The device of claim 28, wherein the control means determines a predicted end-of-life point of the battery by determining a beginning-of-life voltage for the battery and determining that the predicted end-of-life point of the battery will occur when the output signals from the battery monitoring means indicate that the voltage of the battery corresponds to the beginning-of-life voltage.
 31. The device of claim 28, wherein the control means determines a predicted end-of-life point of the battery by determining a peak voltage of the battery based on the output signals provided by the battery monitoring means and then determining that the predicted end-of-life point of the battery will occur when the output signals from the battery monitoring means indicate that the voltage of the battery has a magnitude that is a pre-selected amount less than the peak voltage.
 32. The device of claim 28, having telemetry of greater than 10 kbaud.
 33. The device of claim 27, wherein the delivery means comprises a pacing lead.
 34. The device of claim 27, wherein the delivery means comprises a defibrillation coil.
 35. The device of claim 27, wherein the monitoring means comprises: a band gap reference device coupled to the battery that provides a reference voltage that is substantially temperature independent and substantially voltage independent; and an A/D converter that receives the reference voltage and also receives the output voltage from the battery, wherein the A/D converter sends a digital signal to the control means indicative of the difference between the output voltage from the battery and the reference voltage.
 36. The device of claim 27, wherein the control means develops a normalized battery voltage signal corresponding to each of the output signals wherein the normalized battery voltage signal is processed so as to reduce the effect of temporary variations in the output voltage in the means for supplying power.
 37. The device of claim 36, wherein the control means averages each receives sampled output signals with a pre-selected number of previous sampled output signals to obtain a periodic value for evaluation of whether the periodic value is indicative of the predicted end-of-life of the means for supplying power.
 38. The device of claim 27, wherein the therapy is antitachycardia pacing. 